The present invention relates generally to gas discharge closing switches and, more particularly, to placement of a cathode in such a switch.
Gas discharge closing switches, such as thyratrons, are used to switch large voltages very quickly and with low power loss. A typical thyratron has an anode exposed to extremely high voltages and a cathode held at ground potential. A control electrode or "grid" is placed between the anode and the cathode to close the switch by providing a positive potential which draws electrons from the cathode and generates a dense, conducting plasma by an avalanche process. The onset of the plasma, referred to as "breakdown" of the gas within the device, occurs at a preselected voltage which depends solely on the pressure of the gas and the distance between the control electrode and the cathode. This relationship, known as Paschen's Law, is discussed in depth by Cobine, James D. in Gaseous Conductors, New York, McGraw-Hill Book Company (1941), pp 160-173, the teachings of which are hereby incorporated by reference.
Paschen's Law is often expressed graphically as a "Paschen Curve" relating the breakdown voltage (V.sub.B) to the product of gas pressure (p) and gap distance (d). For a given cathode geometry, this curve has a minimum voltage (V.sub.min) corresponding to a specific optimal value of p*d. The curve itself is empirically determined and is used in designing thyratrons. In this regard, it is often said by workers in the field that the cathode of a thyratron should be located at the Paschen Curve minimum of the device, meaning that the portion of the cathode closest to the control electrode should be spaced from the electrode by a distance which, for the specific gas pressure used, corresponds to the lowest point on the Paschen Curve.
By way of illustration, consider the device of FIG. 1 in which a thyratron 10 has a high voltage anode 12 and a grounded cathode 14 separated by a control electrode or "grid" 16. The distance between the control electrode and the closest portion of the cathode is designated d.sub.1, and the distance between the control electrode and the most distant portion of the cathode is designated d.sub.2. In this context, the curve of FIG. 2 illustrates the typical prior art placement of a thyratron cathode wherein the closest point of the cathode is spaced from the control electrode by the optimum distance d.sub.1. The breakdown voltage (V.sub.B) at the distance d.sub.1 has a minimum value, V.sub.min, which corresponds to the minimum of the Paschen Curve. The more distant points on the cathode, which are spaced from the control electrode by distances between d.sub.1 and d.sub.2, are displaced from the minimum of the curve and therefore have higher breakdown voltages. Under these conditions, breakdown is initiated at the near end of the cathode.
Unfortunately, Paschen Curves are not available for the complex geometries of many current thyratron cathodes. Cathode design based on Paschen's Law has therefore been somewhat imprecise in the past. In addition, when a thyratron breaks down at a point close to the control electrode, the resulting current has a shielding effect that repels current from more remote portions of the cathode. This precludes full utilization of the cathode surface and limits the current density of the device during breakdown.
Therefore, it is desirable in many applications to provide a cathode design for a closing switch which makes full use of Paschen's Law and permits more complete utilization of the cathode surface.